Portable communication devices, such as cellular telephones, use one or more power amplifiers to amplify an information signal prior to transmission. Modern communications systems use advanced modulation schemes, which are both phase and amplitude modulated, to boost information transmission rates, generally at the expense of power consumption. Generally, a linear power amplifier is used for systems that use phase and amplitude modulation (such as systems that employ code division multiple access (CDMA) or enhanced data rates for GSM evolution (EDGE)), while a non-linear power amplifier is used for systems that employ phase only modulation (i.e., a constant envelope modulation system such as Gaussian mean shift keying (GMSK) modulation). A linear power amplifier has significantly lower power efficiency than a non-linear power amplifier. In some communication systems, such as CDMA and wideband CDMA (WCDMA), power consumption is further increased because the linear power amplifier has to operate properly over a wide dynamic range due to CDMA/WCDMA power control requirements. For example, the dynamic range of a WCDMA system can be on the order of −50 dBm to 27 dBm.
The power amplifier is typically implemented as one or more stages of transistors and related circuitry. In most applications, the operating point of the power amplifier is set by providing a bias current or voltage to at least one of the terminals of at least one of the stages of the power amplifier. In the case of a bipolar junction transistor (BJT) the bias current is normally applied to the base terminal of the transistor to control how the transistor will conduct between its collector and emitter terminals. In a typical implementation, the power amplifier comprises one or two driver stages followed by an output stage.
A commonly used method is to control bias current and collector voltage by incorporating a bias current controller and a voltage regulator, sometimes referred to as a “buck-boost” converter. By separately controlling the bias current and collector voltage, the power efficiency of the power amplifier can be optimized.
In some implementations, the radio frequency input to the power amplifier can be provided from a transceiver to one or more of the driver stages. The transceiver is typically located on a separate structure from the power amplifier. In some power amplifier implementations, the driver stage of the power amplifier may contain multiple instances of the driver circuitry so that the power amplifier can be used in more than one communication topology. For example, a single power amplifier device can be implemented to operate in a non-linear topology, such GMSK, and in a linear topology, such as CDMA/WCDMA or EDGE. The non-linear amplifier driver circuitry can be optimized for low power, high gain operation, while the linear amplifier driver circuitry can be optimized for high power, low gain operation.
Unfortunately, the transceiver that drives the power amplifier is typically inefficient at lower power levels, thereby making it difficult for a single transceiver device to operate efficiently with the multiple instances of the power amplifier driver circuitry.
Therefore, it would be desirable to improve the efficiency of a communication system to increase battery time, namely, cellular phone talk time.